if i have my heater off my rpms stay at 900. if i have it on it sways up and down from 900 too 1400. and when its off my voltage tack is like 3/4 of the way to high! have no idea where to start!!!!!

 

Is your AC compressor comming on? would explain the constant shift of idle speed. Shut truck off and measure the voltage drop with the heater motor on and off at the battery cables. The battery should handle the heater motor fairly easily. If you see large voltage drops suspect a weak battery of 5 years of age. Battery will go quickly dead. If you crank the starter for 30 seconds and a dead battery you know.